The present invention relates to a curable fluorine-containing copolymer and a coating liquid composition containing the copolymer.
Hitherto, various fluororesin-containing paints have been produced in an industrial scale, because of fluororesin's superior weather resistance. For example, there have been proposed curable solvent-thinned fluorine-containing copolymers (see Japanese Patent Unexamined Publication Nos. JP-A-57-34107 and JP-A-61-57609). These copolymers have been used for weather resistant paints in various fields such as architecture, automobile, and chemical industries. These copolymers are prepared by copolymerizing main fluorine-containing monomers (e.g., chlorotrifluoroethylene (CTFE), tetrafluoroethylene (TFE), and vinylidene fluoride) with other hydrocarbon monomers (e.g., vinyl esters and vinyl ethers) for increasing the copolymers in solubility. There have also been developed copolymers containing other fluorine-containing monomers. For example, hexafluoroisobutene (HFIB) and hexafluoroacetone (HFA), each of which has one hexafluoroisopropylidene group in the molecule, are known as being able to copolymerize with various other monomers. Thus, it is known that fluorine-containing copolymers prepared by using HFIB and HFA have characteristics that are different from other conventional copolymers prepared by using CTFE and the like. U.S. Pat. No. 3,240,757 discloses that a copolymer of HFIB and another monomer such as vinyl propionate or ethyl acrylate, and a terpolymer of HFIB, vinyl propionate and ethyl acrylate can respectively be dissolved in carbon tetrachloride and toluene. This patent further discloses that a terpolymer of HFIB, vinyl propionate and ethyl vinyl ether can not be dissolved in carbon tetrachloride, methyl ethyl ketone, and toluene, respectively. U.S. Pat. No. 5,053,470 discloses a first copolymer of HFIB and a vinyl ester and a second copolymer having a unit of --CH.sub.2 --CHOH--. It is disclosed therein that coating liquid compositions of these copolymers can provide a coated film that has a superior adhesion to substrate and a superior abrasion resistance, in addition to superior characteristics inherent in fluororesin film. JP-A-60-147415 discloses a coating liquid composition of a copolymer and a curing agent. This copolymer contains 43.0-48.8 mol % of a first structural unit originating from vinyl fluoride, 44.3-49.7 mol % of a second structural unit originating from HFIB, and 4.5-10.4 mol % of a third structural unit originating from hydroxybutyl vinyl ether, glycidyl vinyl ether or an unsaturated alcohol. JP-A-63-218715 discloses another copolymer prepared by copolymerizing 30 mol % of TFE, 20 mol % of HFIB, cyclohexylperfluorovinyl ether, ethyl vinyl ether, and hydroxybutyl vinyl ether. JP-A-64-60616 discloses still another copolymer prepared by copolymerizing 35 mol % of TFE, 15 mol % of HFIB, methyl-2-propenyl ether, chloroethyl vinyl ether, and hydroxybutyl vinyl ether JP-A-8-120213 discloses a further copolymer prepared by copolymerizing 41 mol % of CTFE, 5 mol % of HFIB, a vinyl ester, a hydroxybutylallyl ether, glycerol monoallyl ether, and triethoxyvinylsilane.
Conventional solvent-thinned fluororesin coating compositions of the above-mentioned fluorine-containing copolymers can provide coated films that are superior in weather resistance. These coated films are, however, inferior in stain resistance. In other words, the external appearance of these coated films may be damaged by stains in the form of streaks. In particular, a coated film prepared by using a polyvinylidene fluoride (PVDF) based copolymer is inferior in stain resistance. In order to solve this problem, there is a proposal of increasing the surface hardness of a coated film by increasing the glass transition point of a fluorine-containing copolymer A coated film having the thus increased surface hardness shows good results in the stain resistance test using carbon black or Magic Marker (trade name). Such coated film is, however, still easily stained in the actual exposure test. There is another proposal of adding an additive of metal alkoxide such as alkylsilicate to a fluorine-containing copolymer. With this, the contact angle of water drop on the coated film decreases, and thus the coated film may have rain streaks less than those of coated films prepared by using other conventional coating liquid compositions. In other words, the coated film according to this proposal is improved in stain resistance by increasing its hydrophilcity. It is, however, not preferable to use this coated film for automobile, because the automobile's coated film requires high hydrophobicity. Furthermore, it has been difficult to prepare a coated film that is superior in stain resistance and other characteristics such as recoatability and defoamability, by adding an additive to a fluorine-containing copolymer, as mentioned in the above proposal.